Mistral FF72P fridge vent blocked with ice

Hi

I have a similar issue to most of the posts on Mistral FF72P fridge freezers. The freezer is fine (-22C) but the fridge is at ambient temperature. I took the freezer section apart and the tube connecting the fridge to the freezer was blocked with ice. I defrosted the freezer, put it all back together and the fridge started to chill again.

After this, the fridge temperature oscillated between 4C and ambient for a week. Up until this morning, the fridge was again at ambient temperature. So I again took the freezer apart. The tube between the fridge and freezer was again blocked with ice.

I was going to replace the yellow fridge thermistor, but I was wondering if this is going to be a green evaporator thermistor issue.

Any ideas or am I totally off the mark?

Thanks

Adrian